Both were extremly bad and both costed the U.S billions of dollars to recover from, But which one was the worst..

(New Orleans after Katrina 2005)

(NYC during the 9/11 attacks)

one was a natural disaster that most were warned of, and chose to ignore.
the other was an unprecedented Jihadist terror attack, that killed thousands of unsuspecting people.

also, even though the bottom picture is of a horrible event, it looks amazing, the photographer did a terrific job, it looks unreal.

Katrina, given that it shouldn't have been nearly as bad as it was (due to mismanagement by various parties at every stage), i would class as the worse of the two. 9/11 was harder to foresee, though the Bush Administration was somewhat asleep at the wheel regarding the CIA memos that were circulated, i don't pin as much blame on them for that as some do, given that we were tracking al-Qaeda as best as we could through the late 90s anyway

Depends if you want to look at the events in a vacuum or consider the repercussions of each event.

Where ever there were more deaths...

9/11, I guess, but I'm probably biased by not living in Houston, TX.

I think in terms of cause and effect 9/11 was much worse. It led to a war, and some may argue Iraq war acceptance was a by product of the fear that event caused.

Katrina was a very worse disaster for the people and city.

But 9/11 is the best symbol of the power of some people.

even as the after affects will continue to last for yrs to come although i fill the affect of Katrina will last much longer.

While Hurricane Katrina probably devastated the areas it hit more than 9/11 devastated NYC, 9/11 was far worse on a global scale.
Damages from Hurricane Katrina 81 billion dollars
Total costs of everything that was caused by 9/11 is in the trillions.